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Sea-ice formation
Sea ice maximum, October.
Sea ice around Antarctica varies from about 8 million square miles in September
or October to about 1 million square miles in January or February. This
image from the Nimbus 7 Scanning Multichannel Multiwave Radiometer, which
operated from 1978 to 1987 shows the sea ice maximum in October.
Picture
NASA
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| 1/ What is pack-ice? |
 At
the beginning of the austral winter starting around March, the loose pack
ice that has spent the summer months circling Antarctica begins to drift
northwards.
Pack ice is old sea-ice, frozen sea water that is a year
old or more, it froze and formed elsewhere and later floated off with the
winds and currents.
Pack ice is heavy stuff and when it arrives somewhere
it has the effect of steadying the ocean swell. The continuous rolling motion
of the sea is stopped completely by a relatively narrow band of pack ice
only 100m or so wide. The result is that where pack ice is present in reasonable
quantity, the sea calms down sufficiently for low temperatures to freeze
it easily - moving water cannot freeze as easily as static water.
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| 2/ What is fast-ice? |
 This
is sea-ice in the very early stages of formation. Sea-ice that forms in
situ and attached to the coast is called "fast-ice", it is stuck fast.
In this picture the surface of the sea is beginning to freeze as the temperature
is dropping to -20C and below.
Pack ice has come near to the shore and so all movement
of the sea has been killed completely allowing low temperatures to freeze
the sea water. At this stage the ice is around an inch or 2.5cm thick but
it has a spongy texture, you could poke a finger or certainly a fist through
it relatively easily.
The patterned effect comes from the rise and fall of the
tides. As the tide rises, so the surface of the sea enlarges slightly and
so the ice cracks apart, as the tide falls, so the surface of the sea decreases
slightly and so the slabs of ice overlap at the edges.
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3/ Does the tide
change the way the sea freezes? |
 Sea
ice in the process of forming, the shore of the island in the distance
is about 5 miles (8 kilometres) away and the whole of the sea surface in-between
is made of forming fast ice.
Notice how the slabs of forming ice become larger further
out to sea as there are less undulations of the coast to push the slabs
together as the tide falls.
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| 4/ What are ice
pancakes? |
 The
ice near to the shore here is known as "pancake-ice". This is formed
when slabs of ice that are forming are jostled by the wind and / or movement
of the sea.
The pancakes of ice bash against each other around the edges and start to
curl upwards at the edges
"Pancakes of submerged ice joined with others into
great sheets, the rubbery green ice thickened, an ice foot fastened onto
the shore, binding the sea with the land. Liquid became solid, solid was
buried under crystals."
Annie Proulx - The Shipping News
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5/ Do ice-bergs
move around when the sea freezes? |
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 A
picture taken of consolidated pack-ice.
The ice that you see is mainly pack-ice,
last years ice that formed elsewhere, broke up and floated here. As the
temperature dropped, then this ice became stuck together by fast-ice, sea-water
frozen in situ and attached to the coast that acts as a glue for the loose
bits of pack.
The ice-bergs that you see have been frozen
in position and will remain so until they are freed by the spring break-up
of the surrounding sea-ice.
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| 6/
What is a tide-crack? |
 Once
fast ice (sea-ice frozen in situ) has become established, the patterns of
the earlier pieces disappears. The tide still rises and falls however
meaning that the sea surface expands and shrinks slightly as it does so.
Tide cracks result from this (as ice is not
known for its elastic properties!) that are formed when the ice moves apart,
they close again when the tide falls. A tide crack is often many miles long,
in this case stretching for around 5 miles (8 kilometres), but never more
than about 18", 45cm wide between Signy and Coronation Islands in the South
Orkneys group.
Tide cracks are valuable resources for wild-life as they
provide a region where birds such as snow petrels can fish through for krill
and also as a breathing hole for crabeater and Weddell seals.
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7/ Does pack ice affect shipping? |
 This
is pack-ice in the summer months around the Antarctic peninsula.
The ice looks fairly continuous, but has
quite a lot of open water between the pieces and so can be relatively easily
pushed aside by an ice-strengthened ship, in this case HMS Endurance.
Larger pieces such as this one that are hit by the bow of the ship crack
up into smaller pieces.
Proper Ice breakers have rounded hulls and rounded bows
rather than being sharp and pointed. When breaking through very thick ice,
the front of the ship rides up over the ice and the weight of the ship breaks
through.
Passage is slow though, and heavy on fuel, but most of
all, it takes an experienced and well informed ice-pilot to be confident
in entering such ice so as not to be locked into the pack should the wind
direction change and consolidate the ice.
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8/ What happens to
fast-ice in the summer? |
 At
the end of the winter, rising oceanic swells and increasing temperatures
cause the stable winter sea-ice to break-up and begin to drift away from
where it formed. This years fast-ice
therefore becomes next years pack-ice with a portion of it melting and disappearing
completely. Here a small inflatable zodiac-like craft is (not entirely sensibly)
negotiating quite close, but relatively light pack-ice.
One person drives the boat, while another sits on the
bow pushing the larger pieces of ice out of the way with his feet.
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